This invention relates to thermoplastic injection molding in general, and specifically to a foam injection molding apparatus and process for foam injection molding of large parts useful in various industries.
The injection molding process is one of the most prolific and universally adaptable methods used to produce molded plastic parts of many shapes, sizes, and physical properties that is available today. Unfortunately, when producing pieces over roughly 25 pounds, problems appear which reduce the efficiency of the process. Improving the overall efficiency for producing large structural pieces is one objective of this invention.
Injection molding machines range from a fraction of an ounce injection capacity to very large units that can provide a shotsize over 800 ounces. The total machine process usually involves a machine, plus mold and necessary auxiliary equipment such as material granulating and loading, parts removal, etc.
The typical injection molding machine consists of two basic entities: (1) the injection unit, which converts the cool solid plastic raw material into a viscous liquid by melting the plastic and then pumps it through a tube "runner system" at extremely high pressure (typically 15,000 to 20,000 psi) into the mold and (2) the clamp unit, which carries the fixed and moving halves of the mold. The clamp opens the mold to release the part previously molded then closes and builds clamp pressure against the mold during injection and solidification of the next part.
The predominant injection system today is the screw-type. The present invention is drawn to methods and apparatus for improving the operating efficiency of this type of injection system.
The most widely used methods of operating the clamp mechanism are: (1) toggle type, (2) straight hydraulic and (3) hydromechanical. Clamping force is required to resist the mold's tendency to open up while being injected with high pressure melted plastic. The toggle type utilizes the action of toggle linkages to multiply the force of a small hydraulic cylinder many times. This system is most used on machines from 50 to 500 tons clamp pressure. A straight hydraulic clamp mechanism utilizes a large, full-stroke hydraulic cylinder to open and close the mold and to build clamp pressure. The system is found in all sizes of machines; it is most popular, however, from 200 tons up to the largest available. The hydromechanical-type clamp mechanism utilizes small cylinders to open and close the mold and one or more large diameter, short-stroke cylinders to build full clamp pressure. This type of clamp mechanism has mainly been utilized in machines of 1,000 tons clamp pressure and over.
The method of building full clamp tonnage varies between utilizing one large cylinder at the center of the machine and four smaller cylinders working on the machine's tie rods.
It would be extremely advantageous if an injection molding process could be devised that could reduce the amount of clamping force required to hold the clamps closed. The large amount of clamp pressure requires a great amount of strength in the molds themselves, leading to very large molds. The larger a mold is, the more heat must be dissipated from the mold, as to be discussed below.
Machines are generally sized by five parameters: (1) dimensions of the platens which hold the molds, (2) length of stroke of the platens, (3) clamp force to hold the molds closed (4) injection capacity (the maximum amount of molten plastic that can be injected ("shotsize")), and (5) plasticating rate (the rate the plastic can be melted). Once the part size and number of cavities is established, a layout of the mold can be made and physical size and stroke of the injection machine determined. The other parameters require knowledge of the material, the process, and a large amount of judgment, based mainly on tests performed on the machines.
In a typical molding cycle, the plastic material is prepared and melted, accumulated and then injected or directly injected into the mold cavity, cooled, and removed from the mold. The cycle can be regarded as a large heat exchange system whereby energy is put in at the injection end; the material transferred to the mold where energy is removed (mold cooling). Unnecessary additional heat input at the injection end lengthens the cooling time required, which can be very significant for large pieces produced on such a machine; thus, proper setup is important in obtaining the most productive cycle. Further, in procedures which utilize a typical accumulator, the first melt into the accumulator is not necessarily the first out, and some degradation (and thus waste) of melt is frequent.
Cooling time in general is the largest portion of the overall cycle time, except where very thin wall parts are involved. The direct time elements can be summarized as: (1) mold close and clamp build-up pressure; (2) injection of melted plastic; (3) part cooling; and (4) unclamping and opening of the mold to remove the part.
Many thermoplastic and thermosetting resins can be injection molded. The process is rapid and highly reproducible parts can be achieved. However, the properties of the resin and the characteristics of the injection molding process are extremely important to achieve satisfactory products. Normally, the "melt" (molten plastic) viscosity as a function of temperature is the most important property of the polymer. For most polymer melts, the viscosity is also dependent on shear rate. This is an important property to understand since within a mold cavity, narrow cross-sections can give high shear rates with a resulting change in viscosity.
Injection molded articles generally have superior mechanical properties in the direction parallel to melt flow compared to those perpendicular to melt flow (i.e., anisotropic). This is due to preferential molecular chain alignment. The extent of anisotrophy increases with decreasing melt temperature. Also, inlet melt pressure affects flow rate and usually gives larger anisotrophy in the molded material when increased. Thus, it would be advantageous to operate at higher temperatures and lower pressures. However, the high temperature leads to significantly increased cycle time, as discussed above, since the cooling time is substantially increased.
The production of very large, structurally sound, but lightweight parts are the focus of many manufacturers today. Some industries, such as the automotive industry, use reaction injection molding, or RIM. RIM utilizes a complete processing system comprised of appropriate mechanical equipment and a properly compounded chemical system to achieve fast and economical production of large parts. Pumps capable of very precise volume control are known in this art. High pressure supplies sufficient energy into the materials to permit intimate mixing in impingement mixhead designs which require no solvent or air flushing between shots and can be directly attached to a mold. The machinery, which is capable of high throughputs, can fill large mold cavities, requiring more than 25 or 30 lbs. of elastomeric materials, in extremely short time periods.
These efforts are indeed impressive, but the production of even larger parts is necessary to produce structures such as underground storage tanks, and other large, structurally sound products. In RIM injection molding, the processor is in fact utilizing a complex chemical reaction within the mold unit and consequently must exert a great degree of control over temperature and material flow in order to obtain the necessary reproducibility. A further disadvantage is that control of temperature affects the pumping and mixing characteristics of the ingredients as well as their reactivity. Further, RIM is not suitable for producing large size parts (greater than about 30 lbs.) having great toughness and strength. Thus, techniques other than RIM have been resorted to.
When injection molding a thermoplastic material such as polypropylene, manufacturers have tried to inject gases into the polymer melt so as to control the "blow factor" of the final product. The term "blow factor," as used herein, means the percentage of void space in the final polymer product. For example, for a given volume of 1 lb. solid resin, a 25% blow factor means only 0.75 lb. of resin would fill the given volume. Manufacturers have identified polymer melt pressure, temperature and injected gas content of the polymer melt as critical factors to control the blow factor in the finished product. A slow cooling is necessary for thick-walled parts where a surface skin will harden and trap molten material at the center. If the skin is not thick enough at the time the part is removed from the mold ("ejection"), the part will shrink extensively, distort and harden with large internal voids. If cooling is too rapid, high molded-in stress and warpage of the molded piece will occur. Thus the precise cooling rate must be determined and controlled to reduce cycle time for conventional injection molding of large pieces. In very large parts, e.g., over 30 lbs., there is some cooling of the plastic as the plastic reaches the furthest extremities of the mold. This cooling affects the amount of injection force that is required to completely form the products that are injection molded since viscosity increases proportionately with cooling. Manufacturers have tried to adjust the amount of injected gas to overcome this cooling effect, by expanding gas after the melt is pumped into the mold, but their methods have been less than satisfactory. Frequently the final product wall has a thick outer skin portion which changes abruptly to an internal void region. In other words, although the final blow factor may be precisely as required in percentage of void space, the actual product will shrink extensively and distort or harden with large internal voids, as discussed above.
It would be advantageous to develop an improved foam injection molding method and apparatus which overcome the disadvantages of these methods. Particularly, it would be advantageous to operate a plasticating extruder more efficiently by using it continuously in a foam injection molding process. It would also be advantageous if the molding cycle time could be reduced through efficient mold and mold gate assembly design, reducing part cooling time, which ties up valuable machine time.